feat: Rewrite agentMixin to a helper (#9940)

# Pull Request Template

## Description

This PR will replace the usage of `agentMixin`with the utility helpers
functions.

**Files updated**

1. dashboard/components/widgets/conversation/contextMenu/Index.vue
2. dashboard/components/widgets/conversation/ConversationHeader.vue
**(Not used)**
3. dashboard/routes/dashboard/commands/commandbar.vue
4. dashboard/routes/dashboard/conversation/ConversationAction.vue
5. dashboard/routes/dashboard/conversation/ConversationParticipant.vue

Fixes
https://linear.app/chatwoot/issue/CW-3442/rewrite-agentmixin-mixin-to-a-composable

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

**Test cases**

1. See agent list sorting based on availability, if agents are on the
same status, then sorted by name.
2. Test actions like assigning/unassigning agent from conversation
sidebar, CMD bar, Context menu.
3. Test actions like adding/removing participants from conversation
sidebar.
4. See agent list is generated properly, none value.


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/helper/agentHelper.js

@@ -0,0 +1,83 @@
+/**
+ * Default agent object representing 'None'
+ * @type {Object}
+ */
+export const createNoneAgent = {
+  confirmed: true,
+  name: 'None',
+  id: 0,
+  role: 'agent',
+  account_id: 0,
+  email: 'None',
+};
+
+/**
+ * Filters and sorts agents by availability status
+ * @param {Array} agents - List of agents
+ * @param {string} availability - Availability status to filter by
+ * @returns {Array} Filtered and sorted list of agents
+ */
+export const getAgentsByAvailability = (agents, availability) => {
+  return agents
+    .filter(agent => agent.availability_status === availability)
+    .sort((a, b) => a.name.localeCompare(b.name));
+};
+
+/**
+ * Sorts agents by availability status: online, busy, then offline
+ * @param {Array} agents - List of agents
+ * @returns {Array} Sorted list of agents
+ */
+export const getSortedAgentsByAvailability = agents => {
+  const onlineAgents = getAgentsByAvailability(agents, 'online');
+  const busyAgents = getAgentsByAvailability(agents, 'busy');
+  const offlineAgents = getAgentsByAvailability(agents, 'offline');
+  const filteredAgents = [...onlineAgents, ...busyAgents, ...offlineAgents];
+  return filteredAgents;
+};
+
+/**
+ * Updates the availability status of the current user based on the current account
+ * @param {Array} agents - List of agents
+ * @param {Object} currentUser - Current user object
+ * @param {number} currentAccountId - ID of the current account
+ * @returns {Array} Updated list of agents with dynamic presence
+ */
+// Here we are updating the availability status of the current user dynamically
+// based on the current account availability status
+export const getAgentsByUpdatedPresence = (
+  agents,
+  currentUser,
+  currentAccountId
+) => {
+  const agentsWithDynamicPresenceUpdate = agents.map(item =>
+    item.id === currentUser.id
+      ? {
+          ...item,
+          availability_status: currentUser.accounts.find(
+            account => account.id === currentAccountId
+          ).availability_status,
+        }
+      : item
+  );
+  return agentsWithDynamicPresenceUpdate;
+};
+
+/**
+ * Combines the filtered agents with the 'None' agent option if applicable.
+ *
+ * @param {Array} filteredAgentsByAvailability - The list of agents sorted by availability.
+ * @param {boolean} includeNoneAgent - Whether to include the 'None' agent option.
+ * @param {boolean} isAgentSelected - Whether an agent is currently selected.
+ * @returns {Array} The combined list of agents, potentially including the 'None' agent.
+ */
+export const getCombinedAgents = (
+  filteredAgentsByAvailability,
+  includeNoneAgent,
+  isAgentSelected
+) => {
+  return [
+    ...(includeNoneAgent && isAgentSelected ? [createNoneAgent] : []),
+    ...filteredAgentsByAvailability,
+  ];
+};